Chinese authorities have detained a 16-year-old schoolboy for posting "fabricated facts" on the internet amid an extensive crackdown on the country's relatively free-wheeling online communities.

The boy from Zhangjiachuan county in north-west Gansu province, identified only by his surname, Yang, was detained after rebuking local police on Sina Weibo, China's most popular microblogging service. Local authorities have accused Yang of "picking quarrels and provoking disputes", Chinese media reported.

This summer, Beijing launched a draconian campaign against what it calls "internet rumours", a thinly veiled move to tighten its grip over the censored, yet often surprisingly critical, online communities. On 9 September, China's top court and prosecutor issued a new "judicial interpretation" stipulating that an internet user could be sentenced to three years in jail for posting a "defamatory" message that receives more than 5,000 views or is forwarded more than 500 times.

Yang questioned the local police force's conduct after a corpse was found outside a karaoke parlour earlier this month, according to the Jinghua Times, a Beijing newspaper which first reported the case. The police claimed the man died of a head injury after falling from a high place. On 14 September, Yang wrote on his Weibo page that the man had been murdered.

Yang suggested a coverup: "The karaoke parlour boss is an employee of the court, and there's been a conflict between the police and the masses – they beat up the dead man's family members," said the post, which briefly went viral and was subsequently deleted. The county court said on its own Weibo page that Yang had "fabricated facts", and that the parlour boss was actually a court employee's spouse.

The anti-rumour crackdown has brought down other, more high-profile figures in recent weeks. In late August, authorities arrested Charles Xue, an outspoken Chinese-American venture capitalist with 12m Weibo followers, for soliciting a prostitute. Soon afterwards, Xue appeared handcuffed on state television to apologise for "irresponsibly" posting uncorroborated content to his Weibo page. "Freedom of speech cannot override the law," he said.

Many Weibo users reacted to news of the teenager's detention with outrage. On Friday, user Li Wenbo wrote: "Who is to judge if something is a rumour or not? What kind of institution dares to say its judgment is correct? What if it's wrong?"